This invention relates to a planning tool that assists in assigning frequencies to base stations of a cellular communication system and in identifying critical fixed site receivers that coexist with such a system, and to methods for operating such a planning tool.
In certain circumstances cellular communication systems are required to operate in the same portion on the radio frequency spectrum as other users. This is true for example with personal communication systems (PCS's) that under FCC proposed regulations are required to share the radio spectrum with pre-existing fixed site microwave users. In such situations, the fixed site microwave users in a given geographical area have not completely occupied all available channels within the portion of the radio frequency spectrum available for use, and the cellular system can use this extra capacity without adversely affecting the pre-existing fixed site users.
In many cases, the number, location and operating characteristics of the fixed site users will vary over time, and it may be possible to relocate many of the pre-existing fixed site users out of the spectral region of interest to the cellular system. Thus, the need for spectrum sharing is expected in many cases to fall over time. However, during the period when spectrum sharing is required, the question remains of how best to structure a cellular system to co-exist with pre-existing fixed site users.
The report entitled "Telocator Spectrum Sharing Report: An Overview of Spectrum Sharing Technologies for the Emerging Technologies Band" (Version 2, Telocator Technical and Engineering Committee, Jan. 20, 1993) summarizes seven alternative approaches to the spectrum sharing problem. All of these approaches are directed to cellular systems of the type having multiple base stations which send radio signals to mobile handsets and receive radio transmissions therefrom. Six of the approaches summarized in this report require either the base stations or the handsets to measure microwave interference levels, and then use the measured microwave interference levels in selecting the frequency channels used for communication between the base stations and the mobile handsets.
Such a measurement-based approach brings with it several important disadvantages. First, the base stations, the handsets or both must be capable of measuring microwave interference levels prior to transmitting on a particular channel. This complicates the function and structure of the base stations or handsets, and thereby increases expense. Furthermore, measurement-based approaches which rely on real time measurements of interference levels may require channel reallocation during a radio transmission, if a mobile user's interference to a fixed microwave site increases due to a change in position.
One of the six measurement based approaches described in the above-identified report is identified as the American Personal Communications Frequency Agile Sharing Technology (FAST). This approach utilizes a computer model to predict interference of base stations and handsets with pre-existing fixed site microwave users. This computer model requires measurements to be taken by the base stations and by mobile units to improve the accuracy and to confirm the reliability of the computer simulation. This approach utilizes some of the advantages of a computer modeling system, yet it retains the cost and complexity of a measurement-based system.
The only one of the seven approaches described in the above-identified report that does not require either the base station or the handset to measure interference from fixed site users is the Ameritech Frequency Avoidance by Zone Exclusion (FAZE) system. That system embodies the present invention, and it is described in detail below.